Process of surface treating luminescent materials

ABSTRACT

The luminescent material(s) optionally composed of more color components are suspended in the solution of active agent(s) and treated with 0.5 to 10% by weight of glass forming acid or a mixture thereof, preferably boric acid or phosphoric acid calculated on the weight of the luminescent material. Then a solution comprising 1 to 15% by weight of aliphatic amines calculated on the weight of the luminescent materials is added and the mixture thus obtained is homogenized.

TECHNICAL FIELD

The present invention relates to a novel process for the surfacetreatment of luminescent materials (luminophores) used for theelectrostatic coating of discharge vessels applied especially inlow-pressure mercury vapor discharge lamps.

The luminescent materials treated according to the invention canpreferably be used for coating the envelopes of the new, wide-spread,so-called compact fluorescent lamps. However, they are useful forcoating the envelopes of the usual fluorescent lamps of 38 mm diameterand the envelopes of the energy-saving fluorescent lamps of 26 mmdiameter as well.

BACKGROUND ART

The application of the red, green and blue components of the luminescentmaterials comprising rare earth metals developed for achieving a higherlumen/watt (lumerg) efficiency in the envelope is not difficult by thecommonly used wet methods. The components may be applied in the desiredratio to the inner wall of the discharge vessel by suitably adjustingthe viscosity and density of the coating suspension. Although carbon orcarbonized organic substance contaminations having a disadvantageouseffect on the efficiency of the luminescent material and the gas contentof the discharge tube can remain even after the usual heat treatmentswhen the luminescent material is applied as an organic solution by usingnitrocellulose as binding agent, the disadvantageous effects occure morefrequently when the luminescent material is applied in the form of anaqueous solution. The removal of the binding agent is also moredifficult compared to the removal of nitrocellulose, when theluminescent material is applied in the form of an aqueous solution.

The discharge tubes of 26 mm diameter of the energy-saving fluorescentlamps are generally coated by two coating layers in order to save money.First the aqueous solution of the usual halophosphate luminescentmaterial is applied onto the inner wall of the discharge tube.Thereafter the organic solvent solution of the luminescent materialcomprising rare earth metal is applied. All the disadvantageous featuresof the wet methods occur when this procedure is used.

In order to eliminate the drawbacks of the wet methods, the method ofelectrostatic coating was worked out. This method is used for thepreparation of e.g. low-pressure discharge vessels and described by e.g.U.S. Pat. Nos. 2,426,016 and 4,081,714 and Hungarian patentspecification No. 184,030.

According to U.S. Pat. No. 2,426,016 very finely dispersed silicic acidis used in the course of the electrostatic coating of high-pressuremercury vapor discharge tubes in order to enhance the adhesion betweenthe luminescent material and the glass. However, this compound cannot beused for preparing low-pressure discharge tubes wherein the dischargearc directly contacts the luminescent material as the finely dispersedsilicic acid is disadvantageous from the point of view of lightefficiency and life-time due to its gas permeability and mercury bindingability.

This drawback is aimed to be avoided by U.S. Pat. No. 4,081,714 whichshows a process wherein calcium stearate or -palmitate or stearic acidand palmitic acid together with the nitrate of strontium or calcium areused for the surface treatment and adhesion enhancing of luminescentmaterials, while finely dispersed aluminium oxide is used for assuringthe sprayability. The use of calcium stearate together with calciumnitrate binding agent is disadvantageous from the point of sprayabilityas the latter compound is hygroscopic. The adhesion of the coating hasto be assured by vaporization. In the course of this step the bulbhaving been previously heated to a temperature of 200° C. has to becooled and thereafter wetted by overheated vapor of about 200° C.temperature. The result of the use of the hygroscopic aluminium oxidecan be the same as the use of the finely dispersed silicic acid sincethe absorbed water cannot be completely removed by the heat treatmentsgenerally used in the course of the production of low-pressure mercuryvapor discharge tubes. Also, since the salts of stearic or palmitic acidcannot also completely be removed by the heat treatments, it has to betaken account that some contamination will always remain in the coating.

The present invention is intended to eliminate the drawbacks of theabove-described methods occuring in the course of the production oflow-pressure discharge tubes. Therefore the efforts were directed towardworking out such a coating method which on the one hand facilitates thefaultless electrostatic application of the luminescent material andresults in a perfect quality of the coating, and on the other handassures good adhesion of the coating to the glass by the aid of thebinding agent remaining on the particles of the luminescent material andassures the removal of gas permeable or other disadvantageoussubstances.

SUMMARY OF THE INVENTION

The subject of the invention is a process for the surface treatment ofluminescent materials used for electrostatic coating. In the course ofthe said process the luminescent material or materials optionallycomposed of more components according to the desired color composition,suspended in solution(s) comprising active ingredients, homogenized,dried, powderized and used for electrostatic coating in a manner knownper se.

According to the invention the above aims are achieved when thesuspension of the luminescent material(s) is treated with 0.5 to 10% byweight of a glass-forming acid, preferably boric acid and/or phosphoricacid calculated on the weight of the luminescent material(s). Thereafter1-15% by weight of aliphatic amine(s) calculated for the weight of theluminescent material are added to the homogenized mixture in the form ofa solution, thereafter the mixture thus obtained is homogenized.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention in the course of suspension the luminescentmaterials are treated with acids the anhydrides of which are susceptibleto the forming of low melting glasses. Onto the acid-treated particlesurfaces aliphatic amines are adsorbed. According to the invention notonly halophosphate luminescent materials, but luminescent materialscomposed of two or more components, comprising rare earth metal oxidescan be treated and used for electrostatic coating as well.

The process of the invention has three advantages:

the double acid-alkylamine layer makes the particles hydrophobic, thusthe luminescent material becomes easily sprayable and dischargable

in the course of the heat treatment forming part of the manufacturingprocess the amine is removed and evaporated without leaving anydisadvantageous carbonaceous contaminations

the glass-forming oxide liberating during the heat treatment adheres theparticles together and to the glass.

As glass-forming agents preferably boron trioxide or phosphorouspentoxide can be used as they do not harmfully influence the gas andmercury content of the low-pressure mercury discharge tubes (fluorescentlamps). Boric acid and/or phosphoric acid are applied per se or inalcoholic especially ethanolic solutions onto the surface of theluminescent materials. Then the adsorbed acid coating is transformedinto a "salt" by adding an ester or ketone, especially acetone or ethylacetate, solution of aliphatic amines. As aliphatic amines preferablyamines of 1 to 8 carbon atoms, especially n-butylamine and n-octylamineare preferred.

The double layer thus formed on the particles of the luminescentmaterial imparts a hydrophobic character to the luminescent material.The powder can be easily sprayed, discharged electrostatically and itmelts at a lower temperature which enhances the adhesion of the layer.In order to achieve the appropriate effect, the oxides or the acidswhich can be derived therefrom are applied onto the surface of theluminescent material or a mixture of luminescent materials in an amountof 0.5 to 10% by weight, while the alkylamines are applied in an amountof 1-15% by weight. Care has to be taken of the formation of neutral oralkaline "salts".

The invention is illustrated by the following non-limiting examples.

EXAMPLE 1

Into a flask 150 g of Toshiba SPD5D green luminescent material (granulesize: 1 to 6 μm), 100 g of Toshiba SPD7A red luminescent material(granule size: 1 to 6 μm) and 250 cm³ of alcoholic boric acid solutioncomprising 5% by weight of B₂ O₃ are charged. The suspension ishomogenized by a disk stirrer taking care to avoid cracking of theparticles of the luminescent material. After homogenization 30 cm³ ofn-butylamine dissolved in 250 cm³ of acetone are charged to thesuspension under constant stirring and the homogenization is continued.The slurry thus obtained is poured into a china dish, evaporated anddried in a vacuum drying oven. The dry, solidified substance is groundin a ball or air blast mill into a particle size of 2 to 10 μm.

EXAMPLE 2

Into a baker 10 g of Sylvania blue luminescent material (particle size:1 to 6 μm), 70 g of Sylvania green luminescent material (particle size:1 to 6 μm), 170 g of Sylvania red luminescent material (particle size: 1to 6 μm) and 250 cm³, 3% by weight of phosphoric acid solution arecharged. The procedure of Example 1 is followed and then, thereafter 35cm³ n-octylamine dissolved in 250 cm³ of ethyl acetate are added. Thenthe procedure of Example 1 is followed again. Thus a luminescentmaterial suitable for the formation of a so-called three-strippedluminescent material (luminophore) coating can be obtained.

EXAMPLE 3

A suspension is prepared according to examples 1 and 2 by charging thefollowing components:

250 g of halophosphate luminescent material

100 cm³ of alcoholic boric acid solution comprising 5% by weight of B₂O₃

100 cm³ of ethanol.

To this suspension 20 cm³ of n-butylamine dissolved in 250 cm³ ofacetone are charged. Then the procedure of the preceding examples isfollowed.

Finally a luminescent material suitable for the preparation of thehalophosphate luminescent material coating for bulbs of dischargevessels 38 mm and 26 mm is diameter (energy-saving and normalluminescent lamps) by an electrostatic method is obtained.

The weight ratios of the luminescent materials in the preceding examplesmean a possible embodiment of the invention. According to the colorrequirements, the weight ratio of the luminescent materials(luminophores) can be varied within wide ranges, which is well-known fora man skilled in the art. The weight ratio of the "glass-forming" oxidesin the mixture and the weight ratio compared to each other can also bevaried in accordance with the geometric requirements of the dischargevessel.

We claim:
 1. A process of surface treating a luminescent material whichcomprises treating a suspension of at least one luminescent materialwith 0.5 to 10% by weight of a glass-forming acid or a mixture thereofcalculated on the weight of the luminescent material, and thereafteradding a solution comprising 1 to 15% by weight of at least onealiphatic amine calculated on the weight of the luminescent material. 2.A process as claimed in claim 1 which comprises mixing n-butylaminedissolved in acetone to a suspension of ethanolic boric acid and theluminescent material.
 3. The process as claimed in claim 1 whichcomprises mixing n-octylamine dissolved in ethyl acetate to thesuspension of ethanolic phosphoric acid and the luminescent material. 4.The process as claimed in claim 1 which comprises mixing n-butylaminedissolved in acetone to a suspension of ethanolic boric and phosphoricacid.
 5. The process as claimed in claim 1, wherein the glass-formingacid is selected from the group consisting of boric acid and phosphoricacid and mixtures thereof.
 6. The process as claimed in claim 1, whereinthe glass-forming acid is in an alcoholic solution.
 7. The process asclaimed in claim 6, wherein the alcoholic solution is an ethanolicsolution.
 8. The process as claimed in claim 1, wherein the aliphaticamine is added as a solution in a solvent selected from the groupconsisting of esters and ketones.
 9. The process as claimed in claim 8,wherein the ester is ethyl acetate and the ketone is acetone.
 10. Theprocess as claimed in claim 1, wherein the alkylamine contains from 1 to8 carbon atoms.
 11. The process as claimed in claim 10, wherein thealkylamine is selected from the group consisting of n-butylamine andn-octylamine.